fur/furr << furrow one's brow

The Eggcorn Database has “furl one’s brow” for “furrow one’s brow.” Writers and speakers also have a tendency to change “furrow” into “fur.” It makes some sense: to create the vertical ridges between one’s eyebrows that signal worry/puzzlement/intense concern, the eyebrows have to be drawn together, displaying, especially in the more bushy browed, a face-wide strip of fur.